Leaded avgas will plug up a catalytic converter in fairly short order. It can also foul spark plugs in vehicles, just as in aircraft. The lead is good for valves guides to a degree but lends to valve deposits and inefficiency due to blow-by, as a result.
You may also find that avgas will burn your valves. It can also lead to carburetion issues, and may affect some fuel injection systems.
That aside, avgas is a lot more expensive.
Unfortunately, like using slick 50, many of these things won't be discovered until it's too late. The potential for harm, however, will depend on the engine. Some engines will do quite well, others will require adjustment.